Handle ambiguous diff header, '--- ' can also be present in a minus hunk

`diff -u file1 file2` output starts with '--- file1', a '-- 12' line
being removed results in '--- 12', which was interpreted as the start of
a new diff. Now the number of removed lines announced in the header as e.g.
'@@ -1,4 +1,3 @@' is taken into account for this specific diff input.